Mortality Risk of Juvenile Japanese Sea Cucumber <i>Apostichopus japonicus</i> by the Sympatrically Occurring Hermit Crab <i>Paguristes ortmanni</i>

The density of the hermit crab, Paguristes ortmanni , in the artificial reef of juvenile Apostichopus japonicus increased almost threefold from June to December 2018. Calcareous ossicles of A. japonicus were identified from the stomach contents in 28.8% of the hermit crabs (18.6% in males and 10.2% in females) in December; season hatchery‐produced juveniles are frequently introduced into the wild in Hokkaido, Japan. The mortality rate of juvenile A. japonicus by P. ortmanni was estimated to be 2.5 ± 2.4 individuals day −1 based on laboratory predation experiments. Interestingly, 3% to 5% of individuals survived despite being attacked and injured in all trials, escaping on the shells of hermit crabs. Over 50% of females in the ossicle‐not‐detected group had shield lengths (SLs) smaller than the smallest individual in the ossicle‐detected group. The average SL of the ossicle‐detected group in females was significantly higher ( p &lt; 0.01) than that of the not‐detected group, indicating an increased predation risk for A. japonicus juveniles when larger female P. ortmanni were present. The present study offers new insights into the predatory behavior of P. ortmanni toward A. japonicus juveniles, showing that these sympatric hermit crabs present a considerably high mortality risk to A. japonicus juveniles. It also emphasizes the importance of implementing appropriate measures to protect juveniles from predators during the release process, providing an essential viewpoint for enhancing and rebuilding the wild population of commercially important endangered A. japonicus .